Semiconductor light sources, such as for example light diodes, have been increasingly attracting attention in recent years in particular with regard to their application for illumination purposes. One of the reasons for this is that important technical innovations and major advances have been achieved with respect to brightness, but also with respect to light efficiency (the light output per Watt) of these light sources. Last but not least, it also became possible to develop light diodes which thanks to their relatively long lifespan represent an attractive alternative to conventional light sources such as incandescent (glowing) lamps or gas discharge lamps.
Semiconductor light sources are well known from prior art. Hereinafter, they will be abbreviated as LEDs (light emitting diodes). This term will include in the following text both light diodes that are made from inorganic materials, as well light diodes that are made from organic materials. It is known that the light irradiation from LEDs is correlated with the current flowing through the LEDs. In order to control brightness, LEDs are essentially operated in a mode in which the flow of the current is controlled by the LEDs. For example a switch controller (step-down or buck controller) is preferably used in practice in order to control an arrangement containing one or more LEDs. A similar switch controller is known for example for DE 10 2006 034 371 A1. In this case, a control unit controls a switch which is clocked at a high frequency (for example a power transistor). In the activated state of the switch, the current flows through the LED arrangement and a coil which is charged in this manner. The energy of the coil, which is stored with intermediate storage, is discharged through the LEDs (recovery phase).
The current displays a zigzag form of development through the LEDs: when the switch is in the activated state, the LED current displays a climbing edge, when the switch is turned off, a trailing edge is displayed. The mean value of the time interval of the LED current represents the effective current flowing through the LED arrangement as a measurement of the brightness of the LEDs. The mean effective current can thus be controlled with a suitable clocking of the power switch.
The function of the operating device is in this case to adjust the current flowing through the LED to a desired mean current flow, and to maintain the temporal fluctuations in the range of the variations of the current, which will depend on the high frequency that is used to turn the switch on and off (typically in the range above 10 kHz), at a level that is as low as possible. A wide range of variations of the current (waviness or ripples) exerts a particularly detrimental influence on the LEDs because the spectrum of the emitted light can be changed when the amplitude of the current is changed.
In order to maintain the spectrum of the emitted light as constant as possible, it is known that instead of varying the amplitude of the current to control the brightness of LEDs, a so called PWM (pulse width modulation) method can be used. The LEDs are in this case supplied using the lower frequency of the operating device (typically with a frequency in the range from 100-1,000 Hz) of the pulse width modulation packets at a constant current amplitude (on a time average). The current in one pulse packet is superimposed on the high frequency ripple mentioned above. The brightness of the LEDs can be then controlled with the frequency of the pulse packet, wherein the LEDs can be for example dimmed so that the time interval between the pulse packets is increased. A practical requirement on the operating device is that it should be possible to use the device universally and with as much flexibility as possible, for example independently of how many LEDs representing a load are in fact connected and operated. It should be also possible to change the load during the operation, for example when one LED fails.
Also according to conventional technology, the LEDs are operated in a so called “continuous conduction mode”. This method will be explained in more detail with reference to FIG. 1a and FIG. 1b (conventional technology). In the example indicated in FIG. 1a, a step-down converter (buck converter) serves as a basic circuit for the operation of one LED (or several LEDs connected in series), which is equipped with a first switch S1. Direct current voltage or rectified alternating current voltage U0 is supplied to the operating circuit. The known circuits require an expensive measurement circuit in order to measure the current flowing through the LED during the switched off phase, which can be done for example by measuring the voltage through the LED when the current is turned off. However, a high differential voltage measurement with a high potential is required in this case.
When the first circuit S1 is turned on, energy is built up in the coil L1 (during the time period t_on), and it is then discharged in the turned off state of the first switch S1 (time period t_off) through at least one LED. The resulting current profile with respect to time is illustrated in FIG. 1b (conventional technology). Two pulse packets PWM are indicated in this case. The current profile within one pulse packet is shown at a magnified scale. In order to maintain a constant color, the amplitude of the ripple within the pulse packet should be as small as possible. This can be achieved with a suitable selection of the point in time t0 when the device is turned on, and of the point in time t1, which is the point when the device is turned off. These points in time can be selected for example so that the first switch S1 is activated when the current is below a certain minimum reference value, and so that the switch is turned off when the maximum reference value is exceeded.